1. Technical Field
The present invention relates to liquid crystal displays and methods of manufacturing the liquid crystal displays. The present invention particularly relates to a liquid crystal display containing a liquid crystal controlled by an electric field substantially parallel to a transparent substrate and also relates to a method of manufacturing the liquid crystal display.
2. Related Art
Liquid crystal displays operating in a fringe-field switching (FFS) mode, an in-plane switching (IPS) mode, or another mode are known to have high contrast and wide viewing angles. The liquid crystal displays use electric fields substantially parallel to transparent substrates.
For example, an FFS-mode liquid crystal display includes two transparent substrates and a liquid crystal sandwiched therebetween. One of the transparent substrates has pixel electrodes supplied with display signals. A common electrode is disposed above the pixel electrodes with an insulating layer disposed therebetween. The common electrode includes a plurality of linear portions and slit portions alternately arranged and is applied with a common potential.
FIG. 17 illustrates an example of the arrangement of the pixel electrodes and the common electrode in cross section. With reference to FIG. 17, a planarization layer 18, which extends over a first transparent substrate (not shown) having pixel transistors, underlies pixel electrodes 20 made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). The pixel electrodes 20 are covered with an insulating layer 21 made of an inorganic material such as silicon nitride. The insulating layer 21 underlies a common electrode which is made of a transparent conductive material such as ITO or IZO, which includes a plurality of linear portions 22E and slit portions 22S alternately arranged, and which is supplied with a common potential. The linear portions 22E and the slit portions 22S are covered with a first alignment layer 24 made of a polyimide-based resin. The first transparent substrate is attached to a second transparent substrate (not shown) having a second alignment layer. A liquid crystal LC is sealed between the first and second transparent substrates. The first transparent substrate and the second transparent substrate have a first polarizer (not shown) and a second polarizer (not shown), respectively. The transmission axis of the first polarizer is perpendicular to that of the second polarizer. The rubbing direction of the first alignment layer 24 and that of a second alignment layer are parallel to the transmission axis of, for example, the first polarizer and are planarly inclined at about five to ten degrees to the longitudinal direction of the linear portions 22E.
The FFS-mode liquid crystal display is disclosed in JP-A-2002-296611.
The FFS-mode liquid crystal display, which has higher contrast and wider viewing angles as compared to other liquid crystal displays such as TN-mode liquid crystal displays, has a problem in that the center of an optimum common potential shifts from an initial value during continuous operation and therefore image sticking is caused. This leads to the deterioration of the display quality of the FFS-mode liquid crystal display.
The evaluation of the FFS-mode liquid crystal display by experiments has shown that the shift of the center of the common potential and image sticking significantly depend on properties of the first alignment layer 24.
Suppose attention is focused on members disposed near the first alignment layer 24 shown in FIG. 17. The following interfaces are present on or above the linear portions 22E: the interfaces H between the first alignment layer 24 and the linear portions 22E and the interface I between the first alignment layer 24 and the liquid crystal LC. On the other hand, the following interfaces are present under the slit portions 22S: the interfaces J between the insulating layer 21 and the pixel electrodes 20, the interface K between the insulating layer 21 and the first alignment layer 24, and the interface L between the first alignment layer 24 and the liquid crystal LC. That is, the correlation between layered members disposed on or above the linear portions 22E disagrees with the correlation between layered members disposed under the slit portions 22S.
When electric fields are generated between the pixel electrodes 20 and the linear portions 22E by the difference between the potential of each display signal and the common potential, the amount of charge accumulated at the interfaces H and I on or above the linear portions 22E differs from that at the interfaces J, K, and L under the slit portions 22S. The difference between the charge amounts causes unnecessary direct current components between the pixel electrodes 20 and the linear portions 22E. This probably causes the shift of the center of the optimum common potential and image sticking.
In order to cope with this problem, another material may be used to form the first alignment layer 24 or another liquid crystal may be used instead of the liquid crystal LC. However, this causes trade-off problems such as a reduction in orientation force and image sticking due to excessive charge transfer. Therefore, sufficient improvements have not been achieved yet.